Method and a plant for producing and treating wood fibres

ABSTRACT

The present invention relates to a plant and a method for producing and treating wood fibres in a plant comprising a fibre-producing part having a chip preheater ( 1 ) and a beater ( 2 ) for freeing fibre from wood chips. Such a plant and such a method are included as parts in a continuous production process of board and the object is to reduce the emission discharges of foremost VOC and formaldehyde, during different parts of the process as well as from the finished product. The method comprises feeding of wood chips (A) into an upper part of the chip preheater ( 1 ), discharge of wood chips in the bottom of the chip preheater to a screw conveyor ( 12 ) for further transportation into the beater ( 2 ), compression of the wood chips in a compression zone ( 15 ) in the screw conveyor, supply of steam (B) into the lower part of the chip preheater for washing fed chips in the counterflow direction, disposal ( 8 ) of flue gases in the form of released organic emissions, steam and air in the upper part of the chip preheater, as well as disposal of flue gases generated during the compression through an outlet ( 13 ) arranged in the compression zone. The plant has the corresponding parts.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method for producing and treating wood fibres in a plant comprising a fibre-producing part having a chip preheater and a beater used to free fibre from wood chips, according to the preamble of claim 1, as well as a corresponding plant according to the preamble of claim 9.

BACKGROUND OF THE INVENTION

A continuous production process of board according to the dry and wet method, based on material containing lignocellulose such as wood, straw, bagasse etc., comprises, among other things, a disintegration of the raw material to free fibre or fibre aggregate, which in the subsequent steps is coated with glue, dried, formed and pressed to a finished product, so-called board or wood fibre board. The freeing of fibre from the raw material is today preferably carried out in a so-called thermomechanical process in one step or in a thermal and mechanical processing step in two separate phases.

The thermal part, heating of the raw material, takes place inter alia in an alkalinizing device at a temperature of up to approx. 100° C., under atmospheric pressure, and then in a pressurized chip preheater at a temperature of approx. 150-190° C. under a pressure of approx. 4-13 bar(e). The dwell time in the preheater may be between 1-10 minutes depending on prevailing process conditions and may be adjusted. The thermal heating in the chip preheater preferably takes place by means of steam. The mechanical processing takes place in a beater between beater discs where the final fibre freeing takes place in a state where temperature and pressure is higher than in adjacent process stages. The dwell time of the wood chips raw material in the beater zone is very short. The power that is converted to mechanical energy in connection with the mechanical processing transforms into heat in the beater zone and occurs as steam in the system, generated from the moisture in the raw material.

After the defibering in the beater, the fibre is transported to a pneumatic fibre drier where the drying process is performed by means of a large amount of air and a controlled entering air temperature of approx. 140-200° C., depending on the current fibre moisture included. The dried fibre is then transported further to forming, pre-pressing and finally to final pressing of the board.

According to older prior art, the wood emissions released during the process, foremost in the chip preheater, are transported all the way from preheater via beater together with the fibre bulk to the drier, where the majority is separated from the fibre and finally accompanies humid drying air leaving the drier and out into the atmosphere. These emissions contain, above all, volatile organic substances, so-called VOC (Volatile Organic Compounds), as well as formaldehyde. The remaining amounts, which are not leaving the drier, follows the fibre flow to subsequent process units where they successively are released to surrounding atmosphere or appear as residual product in the finished product, the board. Thus, also from the finished product, discharge of emissions to the atmosphere takes place.

From WO 99/10594 it is, however, previously known to provide the chip preheater with a top outlet for degassing of organic emissions released there. Here, the steam is introduced in the lower part of the preheater and the wood chips, which enter the upper part of the preheater, are washed in the counterflowing steam during condensation. This is achieved by means of the steam moving upwards through the wood chips column towards the colder wood chips in the top of the preheater, and released emissions, air and steam being generated by evaporation of the moisture in the wood chips are separated and disposed of through the outlet into a scrubber. By this publication, it is also known to transport the wood chips from the preheater into the beater by means of a screw conveyor, which also compresses and dewaters the wood chips during the transportation.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method and a plant that reduce the emission discharges, foremost of VOC and formaldehyde, to the atmosphere, from the production process as well as from the finished product, the boards. Simultaneously, the capacity in the production plant should be maintained and so also the quality of the final product.

This object is attained by a method having the new features that are defined in the characterizing clause of claim 1, as well as by a plant having the new features that are defined in the characterizing clause of claim 9.

Thus, the method according to the present invention is characterized in that flue gases generated during the compression are disposed of through an outlet arranged in the compression zone. The plant according to the present invention is characterized in that in the compression zone a flue gas outlet is arranged for disposal of evaporated moisture, which is generated upon the compression of the wood chips and which contains flue gases.

In the compression of the wood chips in the screw conveyor, water is pressed out, which contains a great deal of remaining wood emissions. By providing an outlet in the compression zone, the water can evaporate and the released emissions can be discharged through the outlet. By catching and diverting the emissions that are released in the compression zone directly and on the spot, a considerably higher concentration of the emissions is obtained than if they instead had to be transported further with the fibre bulk and be mixed with the drying air, which represents a very large flow of gas, and thereby provides low concentration, according to prior art. Thus, a considerably more efficient handling of the flue gases from the preheating is obtained by means of the present invention.

According to one embodiment, the disposal of the flue gases that are generated during the compression is achieved by conveying them back to the upper part of the chip preheater from which they then are disposed of together with other flue gases. The flue gases may advantageously be conveyed to destruction, e.g., to incineration in a boiler, where the emissions transform to carbon dioxide and water.

According to another advantageous feature, upon the disposal of flue gases, the flow of flue gases can be controlled, and in this way, also the steam losses in the chip preheater can be controlled.

According to a further embodiment, a so-called catcher agent may be added to the wood chips upon the feeding thereof into the beater. A catcher agent is a chemical that is injected in order to bind some substance, in this case formaldehyde. It is previously known to add catcher agents in other places in the process. However, it is not previously known to add a catcher agent directly into the beater and at tests it has turned out that this is particularly efficient as for binding formaldehyde. Test results with a catcher agent added into the beater have demonstrated a reduction of formaldehyde content by 8-15% and also a clear improvement of perforator values in the final board.

According to a further embodiment, the wood fibres produced and treated in the beater are sent to a drier part and then to a scrubber, and air leaving the drier part, on the way to the scrubber, is conveyed through a heat exchanger for air entering the drier part so that the air to the drier part is heated and the air to the scrubber is cooled. Thereby, the advantage is gained of a certain heat recovery taking place for the air to the drier. Another important advantage is, however, that the air to the scrubber is cooled. Since the condensable hydrocarbons in the emissions originating from the previous stages in the process tend to be aerosols, this makes the purification effect of the scrubber more difficult. The equilibrium relationship between air and water solution of said hydrocarbons is temperature dependent so that at higher temperature in the scrubber, it is difficult to bring the emissions into water phase. For this reason, it is advantageous to cool the air entering the scrubber.

Furthermore, water drops may advantageously be separated in the scrubber by a demister device. Additional cooling by supply of water is also possible.

Thus, by means of the present invention, reduced discharges of foremost VOC and formaldehyde are obtained during the entire production process of board. This results in important environmental improvements, both internally at the production line and externally, i.e., in the environment outside the plant. Furthermore, the advantage is obtained of an improved final product in the form of a board that causes less discharge in the environment where it is used.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail in the form of embodiment examples and reference being made to the appended drawings, in which:

FIG. 1 shows schematically a plant for production and treatment of wood fibres, comprising an embodiment example according to the present invention,

FIG. 2 shows an embodiment example of a first part of the plant according to the present invention, and

FIG. 3 shows an embodiment example of a second part of the plant according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The plant for producing and treating wood fibres, illustrated schematically in FIG. 1, is included in the illustrated example as a part of a continuous process for producing board of material containing lignocellulose.

Such a plant comprises a plurality of stages. The stages that are illustrated schematically in FIG. 1 includes a first stage having a fibre-producing part comprising a chip preheater 1 and a beater 2 for freeing fibres of wood chips, a second stage having a drier part 3 comprising, for instance, a cyclone, and a third stage in connection with the drier part and comprising a scrubber 4, foremost intended for efficient particle separation in connection with the drying. It should be particularly mentioned that the drier part in itself may comprise a plurality of drying steps having a plurality of similar apparatuses and the plant may moreover also comprise additional intermediate steps and details that have not been shown for reasons of clarity.

The fibre-producing part of the plant in FIG. 1 is shown on a somewhat larger scale in FIG. 2. Via an inlet 6, wood chips A are fed into the preheater 1, usually from an alkalinizing bin, which is not shown. In the lower part of the chip preheater, steam B is supplied via steam inlets 7. The chip preheater is also provided with a top outlet 8. In the chip preheater, the wood chips are heated by means of condensing heat from the steam, which is supplied at pressure, temperature and time having been pre-set and adapted to the wood chips raw material. By the fact that the steam B is supplied into the lower part of the chip preheater, it will move up through the wood chips column C towards the colder entering wood chips in the top, and in this way the moisture in the wood chips is evaporated and forms steam containing released organic emissions from the wood. Said emissions, foremost organic volatile substances such as VOC and formaldehyde, are then separated at a high concentration in the top of the preheater via the top outlet 8. Next, from the top outlet, the flue gases travel to some kind of destruction device, which is not shown. The destruction may, for instance, be effected by incineration or by evaporation.

As is seen in FIG. 2, the flue gas line in the top of the preheater is provided with temperature control device, TIC, and the supply line for steam is provided with a pressure control device, PIC. By setting a value of the pressure of the steam that corresponds to a certain temperature, e.g., of the order of 170° C., it is possible, by measuring the temperature of the leaving flue gases, to control these so that unnecessary steam and energy losses are prevented. Thus, the temperature of the flue gases should not exceed the temperature of the entering steam. If this would take place, the flow of flue gases is automatically reduced, in order to thereby prevent that more steam is supplied than what can be utilized in the chip preheater.

In this connection, it may also be mentioned that the degassing in the top of the preheater has a favourable impact on the heat transfer between the steam and the wood chips in the preheater. Normally, the air that is contained in the supplied cold wood chips A reduces said heat transfer, but by means of the present invention this air may instead be directly disposed of through the top outlet 8.

In the bottom of the chip preheater, there is a chip outlet 11. In connection with the same, there is a screw conveyor 12 arranged, which feeds the wood chips to the beater 2. The screw conveyor comprises a compression zone 15 where the water is pressed out of the chip mass and the formed moisture evaporates in the zone so that steam is generated. The compression zone 15 is formed by the shaft, of that part of the screw conveyor 12 that is closest to the beater 2, having an increasing diameter, so that it obtains a conical shape. This results in the chip material in said part being compressed and pushed out against the outer walls of the screw conveyor 12 where a so-called plug flow is formed, which partly plugs up the entrance to the beater. The formed plug separates the beater from the screw conveyor so that steam from the beater cannot travel rearwards into the compression zone. According to the preferred embodiment, a flue gas outlet 13 is arranged in the compression zone so that it is possible to dispose of said evaporated moisture, which contains flue gases. Furthermore, according to the preferred embodiment, said flue gas outlet is connected to a line 14 that conveys the flue gases back to the upper part of the chip preheater 1. Here, they can then be disposed of together with other flue gases via the top outlet 8 and further away for destruction.

In the illustrated example, is also shown how a catcher agent D can be added into the beater 2. A catcher agent is a chemical, which is injected in order to bind some substance, in this case formaldehyde. The member 16 for addition of a catcher agent may, for instance, be formed so that the supply of the catcher agent D takes place via the centre of the screw conveyor 12 into the beater 2. A suitable catcher agent in this case may be urea or the like.

In FIG. 3, an embodiment example is shown of a scrubber plant 4 according to the present invention, which is used in connection with a drier stage. Said scrubber plant may advantageously be used together with the above-described chip preheater 1 and the beater 2, but may also be used together with other types of chip preheaters and beaters and also in other types of plants, and may thus be regarded as an invention in itself. In FIG. 3, a scrubber 4 is shown that receives air E that travels from the drier part 3, via a line 28. Before the same air is allowed to pass into the scrubber, it passes a heat exchanger device 20, for instance, a tube heat exchanger, for the air F entering the drier. In this way, the heat that the air E leaving the drier contains is utilized in order to preheat the air F, G entering the drier, at the same time as the air leaving the drier is cooled down before it enters the scrubber 4. The equilibrium relationship between air temperature and the concentration of formaldehyde will consequently be altered and promotes the transfer of formaldehyde into water phase. The air E leaving the drier may have a temperature of the order of +60° C., the drying air F entering the heat exchanger is as a rule taken from outside and may then have a temperature of +10° C., and the preheated air G entering the drier may then have a temperature of +30° C.

The scrubber is also provided with a number of water sprayers 21, which accordingly spray out water that cools and absorbs the emissions that are in the drying air into water phase. Water sprayers 22 may advantageously also be positioned in the inlet line 26 to the scrubber. In the lower part of the scrubber, there is a water outlet 23, where water is drawn off in order to be conveyed to purification. In the top of the scrubber, there is an outlet 24 for leaving air. The scrubber is suitably also provided with a so-called demister or droplet separator 25 in connection with the upper air outlet, in order to separate water drops.

The supplied spray water may possibly be chemically treated with NaHSO₃ and NaOH in order to optimise the function of the emission reduction. Thus, the scrubber 4 is provided with a device 27 for supply of said additives to the water. Test results have shown that in this way, a 75% reduction of the formaldehyde content in the leaving air is achieved, already without connection of a heat exchanger.

The present invention should not be regarded as limited to the above-described embodiment examples, but may be modified and varied in a multiple of ways, as is realized by a person skilled in the art, within the scope of the accompanying claims. 

1. A method for producing and treating wood fibers in a plant comprising a fiber-producing part having a chip preheater having an upper part and a lower part and a beater for freeing fiber from wood chips, comprising: feeding wood chips into said upper part of said chip preheater, discharging wood chips in said lower part of said chip preheater to a screw conveyor for further transport to said beater, compressing said wood chips in a compression zone in said screw conveyor, thereby generating flue gases therein, supplying steam into said lower part of said chip preheater for washing said wood chips in the counterflow direction, disposing flue gases in the form of released organic emissions, steam and air in said upper part of said chip preheater, and disposing of said flue gases generated during said compressing through an outlet arranged in said compression zone.
 2. A method according to claim 1, wherein disposing of said flue gases generated during said compressing is achieved by conveying said flue gasses back to said upper part of said chip preheater whereby said flue gasses are disposed of together with other flue gases from said chip preheater.
 3. A method according to claim 1 including conveying said flue gases to a destruction device for destruction thereof.
 4. A method according to claim 1 including measuring the temperature of said flue gases disposed of from said upper part of said chip preheater, measuring and controlling the pressure of said steam entering said chip preheater, comparing said temperature and pressure values, and controlling the flow of said flue gases from said upper part of said chip preheater based on said comparison, whereby the steam losses from said chip preheater are controlled.
 5. A method according to claim 1 including adding a catcher agent to said wood chips upon their transport to said beater.
 6. A method according to claim 1, including producing and treating said wood fibers in said beater and sending said wood fibers from said beater to a drier and then to a scrubber, and cooling air leaving said drier before it is treated in said scrubber.
 7. A method according to claim 6, including heat exchanging said air leaving said drier with air entering said drier so that the air conveyed to said drier is heated and the air conveyed to said scrubber is cooled.
 8. A method according to claim 6 including separating water drops in said scrubber by means of a demister device arranged at the air outlet from said scrubber.
 9. Apparatus for producing and treating wood fibers, comprising a fiber-producing portion including a chip preheater having an upper part and a lower part and a beater for freeing fiber from wood chips, said chip preheater having a chip inlet in said upper part of said chip preheater, a chip outlet in the lower part of said chip preheater, a steam inlet in said lower part of said chip preheater and an outlet at the top of said chip preheater for generated flue gases in the form of released organic emissions, steam and air, whereby chips fed into said chip preheater through said chip inlet are washed in the counterflow direction by means of steam supplied through said steam inlet, a screw conveyor for conveying said chips from said chip outlet to said beater including a compression zone, and a flue gas outlet arranged in said compression zone for disposal of evaporated moisture generated upon the compression of said wood chips containing said flue gases.
 10. Apparatus according to claim 9, including a connection line arranged between said flue gas outlet in said compression zone and said upper part of said chip preheater.
 11. Apparatus according to claim 9 including a destruction arrangement for destroying said generated flue gases.
 12. Apparatus according to claim 9, including a temperature control device for measuring the temperature of said flue gases disposed of from the top of said chip preheater, and a pressure control device for measuring and controlling the pressure of said steam entering said steam inlet, and means for controlling the flow of said flue gases from said top of said chip preheater by comparing said temperature and pressure values, whereby the steam losses in said chip preheater are controlled.
 13. Apparatus according to claim 9, including catcher agent addition means for addition of a catcher agent to said wood chips upon said feeding of said wood chips into said beater.
 14. Apparatus according to claim 9, including a drier for drying said wood fibers from said beater a scrubber located downstream of said drier, conduit means for conveying air leaving said drier into said scrubber, and a cooling device for cooling said air entering said scrubber from said drier.
 15. Apparatus according to claim 14, wherein said cooling device comprises a heat exchanger for heat exchange between said air entering said drier air leaving said drier into said scrubber, whereby said air entering said drier is heated and said air conveyed to said scrubber is cooled.
 16. Apparatus according to claim 14 wherein said scrubber comprises water supply means to cool and absorb existing flue gases into a water phase.
 17. Apparatus according to claim 14, including water supply means arranged between said drier and said scrubber for supplying water to cool and absorb existing flue gases into said water phase.
 18. Apparatus according to claim 16 including additive supply means for supplying additives for chemical treatment of said supplied water.
 19. Apparatus according to claim 14, wherein said scrubber comprises a device for the separation of water drops. 